1. Field of the Invention
The present invention is directed to lipoic acid derivatives or salts thereof which selectively kill tumor cells by altering cancer cell metabolism and signal transduction pathways linked to the Warburg Effect, as well as to methods of treating a subject with such lipoic acid derivatives.
2. Related Background Art
All mammalian cells require energy to live and grow. Cells obtain this energy by metabolizing food molecules by oxidative metabolism. The vast majority of normal cells utilize a single metabolic pathway to metabolize their food. The first step in this metabolic pathway is the partial degradation of glucose molecules to pyruvate in a process known as glycolysis which yields two ATP units. Glycolysis can occur even under hypoxic conditions. Pyruvate is further degraded in the mitochondrion by a process known as the tricarboxylic acid (TCA) cycle to produce thirty-six ATP units per glucose molecule, water and carbon dioxide. The TCA cycle requires oxygen. During periods of reduced oxygen levels, normal cells adapt by a variety of mechanisms and return to normal metabolism as oxygen levels are restored. A critical link between glycolysis and the TCA cycle is an enzyme known as pyruvate dehydrogenase (“PDH”). PDH is part of a larger multi-subunit complex (hereinafter “PDC”). PDH, in conjunction with other enzymes of the PDC complex, produces acetyl CoA which effectively funnels glycolysis-produced pyruvate to the TCA cycle.
Most cancers display profound perturbation of energy metabolism. One of the fundamental changes is the adoption of the Warburg Effect, where glycolysis becomes the main source of ATP. An ATP deficit follows reduced TCA ATP generation. In other words, cancer cells behave as if they are hypoxic even when they are not. This change in energy metabolism represents one of the most robust and well-documented correlates of malignant transformation and has been linked to other changes resulting in tumor growth and metastasis. Because of the reduced levels of ATP available as a result of glycolysis largely being de-linked from the TCA cycle, cancer cells increase their uptake of glucose and its conversion to pyruvate in an attempt to make up the energy deficit. Excess pyruvate and other metabolic by-products of the Warburg biochemistry must be managed. A number of these metabolites are known to be cytotoxic, e.g., acetaldehyde. PDC in cancer along with other related enzymes plays a major role in managing and/or detoxifying the excess pyruvate and metabolites. For example, the joining of two acetyl molecules to form the neutral compound acetoin. This generation of acetoin is catalyzed by a tumor-specific form of PDC.
It has been suggested that lipoic acid acts as a cofactor with PDC and related lipoamide using enzymes in detoxifying these otherwise toxic metabolites. Whether lipoic acid is made by healthy and cancer cells or whether it is an essential nutrient is debated in the literature, and both may be the case. The genes required to produce lipoic acid have been identified in mammalian cells. Whether mitochondrial pumps or uptake mechanisms are present in healthy or cancer cells or whether they differ in diverse tissues is not known. Although the TCA cycle still functions in cancer cells, the tumor cell TCA cycle is a variant cycle which depends on glutamine as the primary energy source. Inhibition or inactivation of tumor-specific PDC and related enzymes that detoxify metabolites may promote apoptosis or necrosis and cell death.
Despite extensive work characterizing the highly conserved changes among diverse tumor types and their metabolism, the changes remain to be successfully exploited as a target for cancer chemotherapy. As cancer remains the number two killer of Americans, there is an urgent need for new approaches to disease management. It has been suggested that lipoic acid due to its redox potential properties may be useful in the treatment of diverse diseases involving mitochondrial function such as diabetes, Alzheimers disease and cancer. These reports teach that the availability of the redox shift from SH to S—S be maintained to have the desired effect.
U.S. Pat. Nos. 6,331,559 and 6,951,887 disclose a novel class of therapeutic agents which selectively targets and kills tumor cells and certain other types of diseased cells. These patents further disclose pharmaceutical compositions comprising an effective amount of a lipoic acid derivative according to its invention along with a pharmaceutically acceptable carrier. The present inventors have now discovered additional lipoic acid derivatives beyond the scope of the aforementioned patents.